The basic concepts of a solar tracker are illustrated in U.S. Pat. No. 2,999,943 to Geer. Such trackers employ a pivotal frame, having cannisters at each end which are interconnected at the low ends of the cannisters. Shadow bars overlie the outer half of each cannister to shade the near cannister from direct sunlight when the sun is not directly above the collector surface. A volatile fluid and its vapor is contained with the cannisters, and as the sun moves from east to west, the east cannister is heated to force the volatile fluid into the west cannister, tilting the frame westward to follow the sun. The frame is balanced when equal sunlight falls on each cannister, so the frame points directly at the sun.
The principal advantage of the solar tracker described above are its ability to track the sun in a totally passive fashion. However, considerable difficulty has been encountered in reorienting the frame in the absence of sunlight, usually so that the collector is repositioned for the morning sunrise. U.S. Pat. No. 4,275,712 to Baer refers to a prior application disclosing a system in which the cannisters are painted different colors or otherwise provided with differential insulation. This causes the fluid to be forced from the west cannister to the east cannister at the end of the day because of differential heat loss to reorient the tracker. However, under certain atmospheric conditions, the desired differential heat loss will not be achieved, and the tracker will not reorient itself as desired. A different system is disclosed in U.S. Pat. No. 4,275,712, in which a separate plate is used on the pivotal axis of the tracker to reorient it in the morning. This design adds needless complexity to the system and defeats its basic simplicity. The use of mirrors underlying the east cannister to catch the early morning sun has also been suggested, but this technique is often defeated by early morning cloud cover, a common occurrence in certain locations. These latter two designs are acceptable in many applications, but certain situations require reorientation immediately upon loss of sunlight and such systems are unacceptable.